There are many examples of communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, use OFDM. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period, which can be longer than a coherence time of the radio channel. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols.
To facilitate detection and recovery of the data at the receiver, the OFDM symbol can include pilot sub-carriers, which communicate data-symbols known to the receiver. The pilot sub-carriers provide a phase and timing reference, which can be used to estimate an impulse response of the channel through which the OFDM symbol has passed, to facilitate detection and recovery of the data symbols at the receiver. In some examples, the OFDM symbols include both Continuous Pilot (CP) carriers which remain at the same relative frequency position in the OFDM symbol and Scattered Pilots (SP). The SPs change their relative position in the OFDM symbol between successive symbols, providing a facility for estimating the impulse response of the channel more accurately with reduced redundancy.
In co-pending UK patent application number GB0909579.5 there is disclosed a receiver for receiving a sequence of OFDM symbols transmitted via a channel, and in particular a receiver for receiving OFDM symbols, which have been transmitted in accordance with DVB-T2. Each OFDM symbol comprises a plurality of data bearing sub-carriers on which data is transmitted and a plurality of pilot bearing sub-carriers on which pilot data is transmitted. The pilot sub-carriers are distributed throughout the OFDM symbols of the sequence in accordance with a predetermined pilot sub-carrier pattern. The receiver includes a channel estimator, and the channel estimator includes a pilot data extractor for extracting pilot data from the pilot sub-carriers of each OFDM symbol; a pilot data extrapolator for generating extrapolated pilot data based on pilot data extracted from the pilot data sub-carriers; and a pilot data interpolator operable to process the pilot data by interpolating between the extrapolated pilot data in time and frequency to produce an estimate of the channel. The receiver also comprises a discontinuity detector for detecting a discontinuity in the pilot data processed by the channel estimator, and a controller, which upon detection of a pilot data discontinuity by the discontinuity detector, is operable to provide a control signal to the channel estimator which causes at least one of the pilot data extractor, the pilot data extrapolator and the pilot data interpolator to compensate for the pilot data discontinuity in the pilot data. As such the receiver can be arranged to receive data from OFDM symbols, such as DVB-T2, which include a number of features which may give rise to discontinuities in pilot data extracted at the receiver. In order to accommodate a resulting discontinuity, in the pilot data symbols caused by a discontinuity the receiver is arranged to detect pilot data discontinuities at the receiver and a controller is provided for ensuring that upon detection of a discontinuity in the pilot data, at least one part of the channel estimator is adapted so as to accommodate for the discontinuity.
However it is envisaged that there is a requirement for further improvements in receivers which must cope with discontinuities in pilot sub-carriers.